This study seeks to establish levels of peer homogeneity -sometimes called homophily in the literature- in the use of tobacco, alcohol, and marijuana in friendship relations in early adolescence. Longitudinal data from a representative sample of school population of the Metropolitan Region of Chile was used to identify friendship dyads between students in 7th and 8th grades. Additionally, some estimates about the origin of the observed homophily are provided, as a result of selection processes in which similarity precedes friendship or, alternatively, as a result of influence processes where friendship leads to matching pairs. The results show that initiation into drugs takes place in environments that are moderately homogeneous and that influence effects are more important than selection effects.